Back in April we posted about some new products made by Japanese RTL-SDR experimenter and product manufacturer Nobu. Nobu’s new products were a 1:1 galvanic isolator and a low pass filter. The galvanic isolator isolates the antenna from the RTL-SDR and PC, significantly reducing noise. The low pass filter is useful when used with direct sampling modified RTL-SDRs to filter out any strong interfering signals that are above 14 MHz.

Recently Nobu sent us at RTL-SDR.com some samples of his products. He sent us one of his HF upconverters, a galvanic isolator and a low pass filter.

HF Upconverter

Nobu’s HF upconverter is called the SC-HFCONV-100 and is based on the design by ttrfrech. It uses the ADE-1 frequency mixer from minicircuits, which is the same mixer used by most other upconverters on the market, including the most popular ham-it-up upconverter. It has the following features:

Uses a 100 MHz 50PPM oscillator and will upconvert signals from between 0 – 30 MHz up to 100 – 130 MHz.

It is powered via a USB mini-B port, or a direct 5V connection which is converted to 3.3V via an AMS1117 voltage regulator.

It consumes 30 mA of current.

The input port has static protection via a BAV199 diode

It uses SMA female ports for the input and output.

The PCB measures 38 x 48 mm.

There are low pass filters on board.

In terms of performance the upconverter works well and has similar performance to the ham-it-up, though it appears that the SC-HFCONV-100 has better SNR especially at VLF, but the ham-it-up has better filters. We like it’s small size, but just like the ham-it-up it really needs an aluminium case to protect it. Fortunately Nobu also sells a fitted case.

From the schematic it appears this upconverter uses the ADE-1 mixer in a slightly different way to the ham-it-up. Instead of connecting the antenna to the RF input of the ADE-1 like with the ham-it-up, they connect it to the IF input. This is also the method used in the SV1AFN upconverter, and this method should improve the overall SNR performance when compared to connecting it to the RF input, especially at VLF frequencies. We aren’t sure if there are any disadvantages to this method however. If you are interested, the schematic for the upconverter can be found here and more information about the upconverter can be found here.

The image below shows a comparison between the SC-HFCONV-100 and the ham-it-up on the broadcast AM band. It is clear that the different method of using the ADE-1 used by the SC-HFCONV-100 improves the low frequency response. However, the better filtration in the ham-it-up seems to reduce spurious signals more.

Mouseover the image to see the ham-it-up. (If on mobile tap the image and tap outside the image to toggle it).

Galvanic Isolator

The galvanic isolator uses a Minicircuits TC1-1TX+ 50 Ohm 1:1 0.4 – 500 MHz RF transformer. It has an insertion loss that is below 0.5 dB from 0.4 – 30 MHz. The point of the galvanic isolator is to isolate the antenna ground from the USB ground in order to reduce any ground loops which can cause noise. Note that the antenna connected to the galvanic isolator must have a good ground connection to work properly. The effect of using the galvanic isolator can be quite significant as shown in the test screen shots shown below.

The first image shows reception of a morse code beacon at around 3.5 MHz. With the galvanic isolator the noise floor is reduced significantly and the signal is stronger.

Mouse over the images to see reception without the galvanic isolator.

The second image shows reception of a international broadcast AM station. The test is done in a fairly noisy environment, but without the galvanic isolator lots of extra PC noise is present further affecting the band.

Mouseover the image to see reception without the galvanic isolator.

Low Pass Filter

The low pass filter is recommended to be used with a direct sampling modified RTL-SDR in order to remove interference from strong signals at frequencies above 14 MHz. It is usually not needed with upconverters, as most upconverters should already have a 30 MHz low pass filter or similar built in. Without filtering stations like strong broadcast band FM and pager signals can interfere with HF signals. Unfortunately, we don’t have any direct sampling modified units with us at the moment, so we couldn’t test the filter directly, but we were able to test its performance on attenuating the broadcast FM band. The results show that strong broadcast band stations are significantly attenuated with the filter in place.

Mouse over the image below to see the effect of the low pass filter being removed on the broadcast band frequencies.

Conclusion

In conclusion Nobu’s products work as advertised, and we can definitely recommend them. Nobu’s products are made in Japan, and at the moment can only be bought from the Japanese Amazon store [HF Upconverter – $56 USD] [Upconverter Case ~$25 USD] [Galvanic Isolator – $23 USD] [Low Pass Filter – $~23 USD]. To purchase from outside of Japan you can use a third party shopping service available at http://agent.jzool.com/, which will buy and ship the product to you from Japan.

Disclaimer: Nobu sent us these products free of charge in return for an honest review.

Suppose I connect coaxial cable from a dongle (an unbalanced transmission line) to a balanced antenna (e.g., a simple dipole). As I understand it, this causes problems. Would this “galvanic isolator” solve them?

I suppose that might depend upon where I put it. Let’s say I put it between the coax and the antenna. Would it solve the problem, and if so, would it be serving as a balun?

A balun is named and used for impedance matching, balanced-to-unbalanced, here the balun is used to isolate the two ground planes – the dongle/computer ground from the aerial ground. So a name was chosen to highlight that function 🙂

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What is RTL-SDR

The RTL-SDR is an ultra cheap software defined radio based on DVB-T TV tuners with RTL2832U chips. The RTL-SDR can be used as a wide band radio scanner. It may interest ham radio enthusiasts, hardware hackers, tinkerers and anyone interested in RF.